docs: def disveclayers(self, x, y, layers, aq=None):
          '''Returns two arrays of size len(layers)
          only used in building equations'''
          if aq is None: aq = self.model.aq.find_aquifer_data(x, y)
          qxqy = self.disvec(x, y, aq)
          rv = np.sum(qxqy[:,np.newaxis,:] * aq.eigvec, 2)
          return rv[:,layers]